Monoamine neurotransmitters such as dopamine and serotonin are removed from the synapse by re-uptake using specific plasma membrane transporters. Monoamines may also be degraded by intracellular or extracellular enzymes. In vertebrates, two monoamine oxidases (MAO-A and MAO-B) are important for degrading monoamine neurotransmitters; degradation pathways in invertebrates are varied. C. elegans has several genes with homologies to MAO, including
amx-1 and
amx-2. AMX-2 is the most similar to vertebrate MAO, while AMX-1 is more similar to the highly related vertebrate flavin-containing amine oxidase domain containing protein 1. Other genes include
amx-3 and F55C5.6 (both similar to polyamine oxidase 1), C24G6.6 (similar to PAOX peroxisomal oxidase), and T08D10.2 and
spr-5 (both similar to lysine-specific histone demethylase). The C. elegans Gene Knockout Consortium has isolated deletions of
amx-1,
amx-2, and
amx-3. We have out-crossed all three mutants, made double and triple mutants, and are characterizing their monoamine levels and behavior. None of the genes have reported RNAi phenotypes. Preliminary results with induced fluorescence show slightly elevated dopamine levels in the dopaminergic neurons in the head in
amx-2;
amx-1 double mutants and slightly higher serotonin levels in the NSMs in
amx-3. The mutants show even higher levels of monoamines compared with N2 after soaking in dopamine or serotonin. Pamx-1::GFP is found in the nervous system (in more than the monoaminergic neurons) and in the early embryo; Pamx-2::GFP expression appears in the gut and neurons; and we have not yet characterized Pamx-3::GFP expression. Behaviorally,
amx-1,
amx-2, and
amx-2;
amx-1 mutants are healthy, while
amx-3 appears slightly slow growing and thrashes at a slightly slower rate.
amx-1 shows minor abnormalities in behavior, including a surprising resistance to DA-dependent inhibition of movement. We have just made the triple
amx-2;
amx-1;
amx-3 and will be characterizing its behavior and sensitivity to serotonin and dopamine. Finally, we are constructing mutants that contain
cat-1 as well as one or more of the putative monoamine oxidases.
cat-1 mutants lack the vesicular monoamine transporter and also show low levels of dopamine and serotonin in the monoaminergic cells, possibly because endogenous enzymes degrade the monoamines if they are not sequestered in vesicles. If this hypothesis is correct and if the amx genes encode MAOs, then
cat-1 double or triple (or quadruple) mutants with amxs may show higher levels of monoamines.